This application relates to a synchronous compensator for a variable speed motor drive.
Modern variable speed motor drives typically use a rotor position sensor for determining the position of the motor rotor, and for commutating inverter switches from one pattern to the next. A device called a resolver is commonly used to sense rotor position and provide feedback. Systematic errors in the determination of the rotor position introduce errors into a variable speed motor drive speed and a current loop, and can produce undesirable operational characteristics.
The resolver error can introduce rotor synchronous errors into position output that occur at harmonics of once per resolver electrical revolution. This is also once per mechanical revolution for a two-pole resolver, and P/2 per mechanical revolution for a P-pole resolver.
These synchronous resolver errors can introduce synchronous speed and current oscillation into the variable speed drive operation. This can result in undesirable cyclic variations in input current and/or electrical power draw. Cyclical variations in the input current and power draw can reduce a variable speed drive's performance. Also, such variations can violate electrical load power quality requirements, like current modulation, in tightly specified load equipment applications such as those found on aircraft.
Additionally, variable speed drives are often used to drive loads that are cyclic in nature. As an example, positive displacement pumps or compressors have cyclic torques associated with their loads, which can cause cyclic speed and current pulsations. This can also result in input current and power oscillations at the variable speed drive that may reduce system performance. These pulsations can also violate electrical load power quality requirements, like current modulation, in tightly specified load equipment applications such as those found on aircraft. That is, load power quality is measured at the load input electric terminals. It can be violated if the above-mentioned pulsations cause input current and power oscillations that violate a specification.
Speed and current control loops for a variable speed drive have been designed and tuned to meet a multitude of system performance requirements. As an example, transient response, stability margins, electrical input impedance, overall efficiency, and steady state accuracy are all addressed. The resulting control loop architectures and associated parameters may unintentionally actually make the sensitivity of the variable speed drive worse to resolver synchronous errors, and to cyclic or pulsating loads.